1. Field of the Invention
The present invention relates to an image forming apparatus, such as an electrophotographic printer. More particularly, the present invention relates to an image forming apparatus that forms an image with liquid developer and a method thereof.
2. Description of the Related Art
Generally, an image forming apparatus, such as an electrophotographic printer, forms an electrostatic latent image on a photoconductor, such as a photoconductive belt or an organic photoconductive (OPC) drum. The latent image is developed with developer having a predetermined color. The developed image is transferred onto a sheet of record paper, thereby obtaining a desired image.
Such an electrophotographic image forming apparatus is classified into a wet type or a drying type depending on the developer employed therein. A wet type electrophotographic image forming apparatus uses a liquid developer formed by mixing powdered toner with a liquid carrier having volatile components as the developer.
FIG. 1 shows a conventional wet type electrophotographic color printer using a liquid developer.
As shown in FIG. 1, the wet type electrophotographic color printer 1 includes an image forming unit 5 and an image transfer unit 10.
The image forming unit 5 includes four image forming units, for example Y, M, C, and K image forming units, to form an image of four colors, that is, yellow (Y), magenta (M), cyan (C) and black (K).
Each of Y, M, C, and K image forming units is provided with a photoconductor 9 having a surface on which an electrostatic latent image is formed. An electrification roller 12 is disposed adjacent to the photoconductor 9 for electrifying the surface of the photoconductor 9 with a predetermined electric potential. A laser scanning unit 11 emits a light beam onto the electrified surface of the photoconductor 9 to form the electrostatic latent image thereon.
Below the photoconductor 9, a developing device 13 is disposed for developing the electrostatic latent image with liquid developer 48 having predetermined color, that is, Y, M, C, or K and a density in the range of, for example, 3 through 20% solid, thereby forming a developer image 49 (see FIG. 2) having a density in the range of, for example, 20 through 25% solid.
The image transfer unit 10 includes four first image transfer rollers 8, a second image transfer roller 23, and an image transfer belt 17. The image transfer belt 17 rotates along a path of endless track on a support roller 21 driven by a belt driving roller 22. As shown in FIG. 2, each first image transfer roller 8 applies predetermined voltage and pressure to the developer image 49 of Y, M, C, or K formed on corresponding photoconductor 9 to form developer image 49′ having a density in the range of, for example, 25 through 30% solid, and at the same time transfers the formed developer image 49′ onto the image transfer belt 17. The second image transfer roller 23 transfers the developer images 49′ transferred onto the image transfer belt 17 to an image receiving medium P, such as a sheet of record paper.
According to the conventional printer 1 configured as described above, when the developer images 49 formed on the respective photoconductors 9 are overlappingly transferred onto the image transfer belt 17 by the voltage and pressure of the respective first image transfer rollers 8, they are squeezed at transfer nips between the respective photoconductors 9 and the image transfer roller 17 by the pressure of the respective first image transfer rollers 8. As a result, the density of developer images 49 is changed from 20 through 25% solid to 25 through 30% solid.
At this time, however, at inlet sides of the transfer nips between the respective photoconductors 9 and the image transfer roller 17, liquid carrier 48′ (referred as “squeezed carrier” below) is squeezed and generated from the developer image(s) 49′ which is or are previously transferred onto the image transfer belt 17 and/or the developer image 49 which is newly transferred thereonto, and accumulated. The accumulated squeezed carrier 49 affects the developer image(s) 49′ that is or are previously transferred onto the image transfer belt 17 and/or the developer image 49 that is newly transferred thereonto, thereby producing image defects.
More specifically, for example, when a developer image 49 (referred as “M developer image” below) of the photoconductor 9 (referred as “M photoconductor” below) that is at a second position from the leftmost side in FIG. 1 is overlapped and transferred onto a developer image 49′ (referred as “Y developer image” below) previously transferred onto the image transfer belt 17 from prior photoconductor, that is, a photoconductor 9 (referred to as “Y photoconductor” below) that is at the leftmost side in FIG. 1, a squeeze carrier 48′ is squeezed and generated from not only the newly transferred M developer image 49 but also the previously transferred Y developer image 49′, and accumulated at an inlet side of transfer nip between the M photoconductor 9 and the image transfer roller 17. As a result, the newly transferred M developer image 49 and/or the previously transferred Y developer image 49′ are affected by the squeeze carrier 48′. Therefore, image defects, such as flow pattern, image dragging and the like, result from an increase in the amount of carrier that may be produced as the developer images are overlappingly transferred onto the image transfer belt 17.
Such an image defect due to the squeeze carrier 48′ is produced more severely at the posterior transfer nip rather than at the prior transfer nip. The reason is because at the posterior transfer nip, the newly transferred developer image 49 is squeezed along with the developer image 49′ previously transferred at the prior transfer nip as the developer images 49 of the respective photoconductors 8 are overlappingly transferred onto the image transfer belt 17. The squeezed carrier 48′ accumulated at the inlet side of the posterior transfer nip includes a squeezed carrier 48′ squeezed from the developer image 49′ previously transferred at the prior transfer nip as well as the newly transferred developer image 49.
Also, the more print, that is, the amount of the developer images 49 transferred to the image transfer belt 17 from the respective photoconductors 9, the greater the image defects produced due to the squeeze carrier 48′. The reason is that the more the amount of the transferred developer images, the greater the amount of the squeezed carrier 48′ accumulated at the transfer nip.
Accordingly, there is required an image forming apparatus that when the developer images are overlappingly transferred onto the image transfer belt 17 from the respective photoconductors 9, the squeezed carrier 48′ in liquid state is not accumulated at the inlet side of the respective transfer nips beyond a predetermined limit, thereby preventing image defects from being produced.